When we think of fish tails, we usually think of the caudal fin. But, the “tail” of a fish is much more than just the last bit. Like a cat, the tail starts just behind the vent and includes all the vertebrae, muscle, skin, and spines from that point to the tip of the fin. At some point in their lives, most fishes use their tail for locomotion. Even pectoral fin swimmers like poachers (Agonidae) and surfperch (Embiotocidae) use their tails for quick movements. Despite their seemingly similar function in propulsion, the morphology of fish tails is extremely diverse from the internal structure of the vertebrae to the external shape and composition of the skin. All fish tails have the same base components. We can model how the shapes of these components result in different material properties and swimming kinematics, in order to answer broad questions about tail evolution and function. We started by measuring the 2D vertebral morphology of the tails of 80 species of fish. We then created a model to translate how the 2D shape of the intervertebral material relates to the 3D structure. To develop an understanding of the mechanics, we measured bending stiffness of whole fish tails, tails with components removed (skin, muscle, etc), and isolated vertebral joints. Finally, we measured the swimming kinematics of 20 species of fishes. Using these data, we created a model showing how the different shapes of the tail vertebrae across the fish tree leads to the diversity of tail function in fishes. This model can be applied to fishes not used in this study to address questions of function throughout evolutionary history.